Japanese Laid Open Patent Application No. 2009-9495 describes a vehicle driving supporting system, wherein based on the steering operation, a long-time running state distribution corresponding to an overall driving characteristics and a short-time running state distribution corresponding to a current driving characteristics are computed; from the magnitude of a difference between the two computed distributions, an instability of the driving state is determined. According to the description in Japanese Laid Open Patent Application No. 2009-9495, this system can detect the instability of the state of driving with a high degree of precision and independent of the surrounding traffic.